Gas flow in the shear layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the surface: essentially zero at the surface, raising to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the gas times the square of its velocity. Along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case on the suction side of an airfoil (such as a fan blade or helicopter blade), the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. The momentum and energy of the gas along the surface is consumed in overcoming the pressure rise and friction so that the gas particles are finally brought to rest and the flow begins to break away from the wall, resulting in boundary layer separation, downstream of the separation point. Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift) and dramatic decrease in system efficiency, due to conversion of flow energy into turbulence, and eventually into heat. It is known that boundary layer separation can be deterred or eliminated by increasing the momentum flux of the gas particles flowing near the surface. In the art, the deterrence or elimination of boundary layer separation is typically referred to as "delaying the onset of boundary layer separation".
One method for overcoming boundary layer separation is simply blowing high energy gas tangentially in the downstream direction through a slot to directly energize the flow adjacent to the surface. This technique, however, requires a source of pressure and internal piping from the source to the orifices at the surface. This greatly increases the cost, weight and complexity of any such system, and have not as yet been found to be sufficiently effective to warrant any practical use.
In the helicopter art, it is known that retreating blade stall establishes limits on rotor load and flight speed. In addition to the loss of capability to generate lift, unsteady blade stall transmits very large impulsive blade pitching moments to the flight control system.
In order to prevent excess control loads, stall boundaries are set as a function of rotor load and flight speed. Stall boundaries define the maximum blade loads, which impact maneuverability and agility as well as speed and payload. Improved payload capability can arise from gains in aerodynamic efficiency in hover via reduction of tip stall and in forward flight via reduction in retreating blade stall. In axial flow, gas turbine engines, such as those used in military aircraft and in commercial transport aircraft, a totally different problem is fan blade wake blockage at the entrance to the core region (low compressor) of the engine. This occurs near the root of the blade.
Yet another problem in any fan is blade tip leakage. To date, no scheme has been found to solve these problems which does not ultimately degrade overall engine performance, due to energy consumed by the compensating apparatus, or parasitic impact on the overall system.